1. Field of the Invention
The present invention relates to an abnormality determination device for an air-fuel ratio sensor.
2. Discussion of the Background
A conventional abnormality determination device for an air-fuel ratio sensor of this type is disclosed in, for example, Japanese Patent No. 2812252. The air-fuel ratio sensor is of the so-called inverse type having an output characteristic that the output value rapidly changes before and after an exhaust gas air-fuel ratio corresponding to the theoretical air-fuel ratio of the air-fuel mixture. In this abnormality determination device, a differential value of an output value of the air-fuel ratio sensor is calculated, and the number of times the differential value becomes less than or equal to a predetermined value is counted. If the count value obtained during a predetermined period is below a predetermined value, the output value is regarded as being stagnant (the sensor is regarded as malfunctioning) and it is determined that an abnormality has occurred in the air-fuel ratio sensor.
Another conventional abnormality determination device for an air-fuel ratio sensor is disclosed in Japanese Unexamined Patent Application Publication No. 2003-20989. The air-fuel ratio sensor has an output characteristic that the larger the exhaust gas air-fuel ratio, that is, the leaner the exhaust gas air-fuel ratio, the larger the output value. In this abnormality determination device, a maximum value of a differential value of an output value of the air-fuel ratio sensor is calculated when normal operation resumes after the termination of the fuel-cut operation that stops the supply of fuel to an internal combustion engine. When a state where the maximum value is smaller than a predetermined value occurs twice consecutively, the output value is regarded as being stagnant and it is determined that an abnormality has occurred in the air-fuel ratio sensor.